The invention relates generally to methods and apparatus for gaging the volume of liquid in a container. More specifically, the invention relates to improving the accuracy of such methods and apparatus by compensating volummetric measurements for inaccuracies that are dependent on the volume of liquid in the container and attitude variations of the container.
Many different types of liquid gaging apparatus are well known. For example, capacitive fuel level sensors have been used for many years as part of fuel gaging apparatus for aircraft fuel tanks. Other types of sensors for fluid gaging include resistive, acoustic and optic, for example.
In the example of fuel gaging apparatus for aircraft, such systems have been installed on some airplane models for as many as twenty to thirty years. As original equipment, these systems were state of the art and have served well the needs of airplane designers and users for many years. A typical system includes profiled capacitive tank probes and electromechanical servo driven cockpit fuel gages and indicators. Any number of probes may be installed in a given tank depending on the accuracy desired, attitude range, complexity of the tank geometry and the need to be able to obtain accurate readings when one or more probes fails to operate properly.
Such probes typically operate by detecting the level or height of fuel in the tank at each probe location. By knowing the geometry of the tank, and the fuel level within various regions of the tank, the total volume (and hence mass when the density is known or approximated) of fuel can be determined, as is well known. For example, with a vertically oriented capacitive probe, the probe capacitance can vary in relation to the percent immersion of the probe in the fuel. By "profiled" is simply meant that the output of a particular probe is adjusted to take into account the different shapes and profiles that a fuel tank may have.
Because of the high replacement cost and long lead times for acquiring new aircraft, and further due to the proven reliability of many of the older aircraft in use today, the current practice of the commercial airlines and military is to continue to keep these older airplanes in service by performing system upgrades to extend the airplane's useful life. Even a complete aircraft overhaul can be significantly less expensive than the purchase of a new or recently built airplane.
Such upgrades can involve any or every aspect of an airplane design from structural repairs and engine replacements to installing more modern avionics equipment. These upgrades often involve the fuel quantity gaging equipment where there is a desire for improved accuracy in fuel quantity measurement. Often, such performance upgrades include the desire to replace the old cockpit electromechanical indicators with more reliable digital indicators, such as for example, an LCD indicator.
The general criteria to enable upgrading airplane designs are cost (including replacement and maintenance), preferably minimal aircraft modifications (for example, a new piece of equipment typically will have to be a direct form, fit and function replacement of the existing system), and improved system performance.
The present invention is directed to improving accuracy of existing and/or installed liquid gaging systems, such as for example, airplane fuel gaging equipment, without having to discard or replace the basic components of such systems as has normally been the case heretofore. The invention thus enables an existing liquid gaging system to continue to use the same profiled probes, for example, while achieving a substantial improvement in fluid gaging accuracy, simply by replacing, for example, the fuel quantity indicator. The invention thus avoids having to remove the actual tank probes and replacing them with new and possibly additional probes as typically would be the case heretofore.